β-glucans are polymers of glucose derived from a variety of microbiological and plant sources including, for example, yeast, bacteria, algae, seaweed, mushroom, oats, and barley. Of these, yeast β-glucans have been extensively evaluated for their immunomodulatory properties. Yeast β-glucans are represented in various forms such as intact yeast, zymosan, purified whole glucan particles, solubilized zymosan polysaccharide, or highly-purified soluble β-glucans of different molecular weights (1-11). Structurally, yeast β-glucans are composed of glucose monomers organized as a β-(1,3)-linked glucopyranose backbone with periodic β-(1,3) glucopyranose branches linked to the backbone via β-(1,6) glycosidic linkages. The different forms of yeast β-glucans can function differently from one another. The mechanism through which yeast β-glucans exert their immunomodulatory effects can be influenced by the structural differences between different fauns of the β-glucans such as, for example, its particulate or soluble nature, tertiary conformation, length of the main chain, length of the side chain, and frequency of the side chains. The immune stimulating functions of yeast β-glucans are also dependent upon the receptors engaged in different cell types in different species, which again, is dependent on the structural properties of the β-glucans.
Complement proteins in the serum, specifically C3 and C1q, are serum pattern recognition receptors (PRRs) that recognize pathogens and are subsequently activated by the classical, alternative, or lectin pathways (Brown G D 2006). All three share the initial step of C3 activation and breakdown into C3a and C3b proteins, and the final step of forming the cytolytic membrane attack complex (MAC) consisting of complement proteins C5b, C6, C7, C8, and C9 (C5b-9). After the initial break down of C3, C3b covalently associates with the carbohydrates or proteins present on the surface of pathogens. This initial process of C3b attachment to the pathogens is followed by further breakdown of bound C3b into iC3b, C3c, and C3dg fragments which ultimately lead to complete opsonizing of the pathogen (Reviewed in Walport 2001). Complement opsonizing of pathogens can lead to either direct killing of the pathogen by forming the MAC or to indirect recognition and destruction of the pathogen by opsonic receptors such as complement receptors on the leucocytes. Zymosan, a crude particulate β-glucan obtained from cell walls of Saccharomyces cerevisiae has been well known for years as the stimulator of the antibody independent alternative pathway of complement activation (Czop J K 1985, Fearon D T 1977, Pillemer 1954, Pillemer 1956).